1. Field of the Invention
The present invention relates to an optical imaging device for irradiating low coherent light to an object, and constructing tomographic images of the object using information-bearing light scattered from the object.
2. Description of the Related Art
In recent years, optical coherent tomography (OCT) for constructing tomographic images of a tissue using low coherent light has been proposed in, for example, U.S. Patent Publication No. 5459570 and Patent Publication No. WO98/52021 as a modality for optically detecting and visualizing information of a living tissue to assess a lesion in the living tissue.
According to the U.S. Patent Publication No. 5459570, a reference mirror is advanced or withdrawn in order to detect light scattered or reflected from a specific depth in a living tissue. For constructing tomographic images of the living tissue, a light beam is irradiated to the living tissue for the purpose of scanning. Synchronously with the scanning, the reference mirror is advanced or withdrawn.
On the other hand, the patent publication No. WO98/52021 has proposed an optical coherent tomography (OCT) system capable of being driven with a voltage having several tens of kilohertz. According to a method described in the patent publication, a diffraction grating is used to disperse the spectrum of light for the purpose of detecting light scattered or reflected from a specific depth in a a living tissue. A galvanometer mirror or an acoustooptic modulator (AOM) may be used to irradiate light for the purpose of scanning, whereby the phase of reference light and the group velocity thereof are changed.
However, the U.S. Patent Publication No. 5459570 has revealed that since the reference mirror is relatively heavy, the frequency of a voltage to be applied to drive the reference mirror so as to advance or withdraw it by about 5 mm is limited to several tens of hertz. A continuous motion picture cannot therefore be produced. This discourages diagnosis of a living tissue in that image quality is poor due to blurring deriving from motions including heartbeats.
A Michelson interferometer may be employed. In this case, an optical coupler works most efficiently while offering a branching ratio of 1:1. Assuming that the power of a light source is P and the reflectance of light from an object is R, light returning to a detector is expressed as Pxc3x97R/4. Assuming that the reflectance for a mirror is 1, the amount of light returning to the detector over a reference light path is expressed as P/4. The amount of light finally returning to the detector is expressed as (Pxc3x97R/4+P/4). However, signal light that must be detected is detected through heterodyning and therefore expressed as (Pxc3x97R/4xc3x97P/4)=P(R/4). The reflectance R observed in a living body is approximately 10xe2x88x924 or less in general. The signal light is therefore very small for the amount of light returning to the detector. It is therefore hard to improve a signal-to-noise ratio. Moreover, 75% of feeble light reflected from a living body is abandoned. This also degrades the signal-to-noise ratio.
U.S. Patent Publication No. 3565335 has disclosed as a method for improving the signal-to-noise ratio using the Michelson interferometer. According to the disclosed method, light that returns to the detector is attenuated to the same extent as signal light by disposing an attenuator on the reference light path, and thus the amount of light returning to the detector is adjusted. However, this method has a drawback that light detected through heterodyning is also attenuated. The U.S. Patent Publication No. 3565335 has disclosed adoption of a Mach-Zehnder interferometer as a method superior in principles to the method of adopting the Michelson interferometer. However, the Mach-Zehnder interferometer is structured to move a corner mirror serving as an optical length varying means. In this case, it is hard to rapidly scan an object in a direction of its depth and observe the object in real time.
Furthermore, in the Michelson interferometer, up to a quarter of source light returns to the light source over the reference light path. The return light causes destruction of a low coherent light source realized with a super-luminescence diode (SLD) or the like. The Michelson interferometer has a drawback that an expensive isolator or the like is usually needed to treat light of wavelengths falling outside a wavelength band assigned to optical communication (1.3 or 1.55 xcexcm).
Furthermore, since the Michelson interferometer employs optical fibers, a polarization controller or the like must be used to match polarization of object light, that is, light to be irradiated to an object with polarization of reference light, that is, light used as a reference. This is mandatory to produce coherent light of a maximum power. However, assume that a reflection type rapid light delay line like the one described in xe2x80x9cIn Vivo Video Rate Optical Coherence Tomographyxe2x80x9d written by A. M. Rollins et. al (Optics Express, Vol. 3, No. 6, P219, 1998) is used in combination with a device having the property of causing polarization such as a diffraction grating. In this case, an incidence optical fiber and an emission optical fiber are identical to each other. Even when the reference light path, object light path, and polarization controllers lying on the reference light path and object light path respectively are adjusted, polarization of the reference light is not always matched with that of the object light with the use efficiency of the reference light held high. There is a possibility that only coherent light of a low power can be produced.
Furthermore, when the reflection type rapid delay line is adopted, light reflected from an end of an optical fiber or the surface of an optical device other than a movable mirror serves as return light. Noise light other than signal light required may therefore be generated. This also deteriorates the signal-to-noise ratio.
Moreover, when a reference arm is employed, a mirror is displaced rapidly relative to light in order to change the phase and group velocity of light. At this time, a phase change causes a Doppler shift. Therefore, when signal light received by a photodetector is detected through heterodyning, an interfering signal can be detected highly sensitively.
However, for rapidly displacing a galvanometer mirror or the like, it is necessary to drive the galvanometer mirror with a voltage proportional to the sine of an angle of displacement. In this case, the Doppler shift occurs at a rate proportional to the cosine of the angle of displacement that is regarded as the derivative of the Doppler shift to the angle of displacement. Moreover, a heterodyne frequency to be detected varies. Consequently, the signal-to-noise ratio is degraded. Otherwise, since light undergoing little Doppler shift is detected, the efficiency in detection is deteriorated.
An object of the present invention is to provide an optical imaging device having a rapid reference scanning means that offers a high signal-to-noise ration and enables realization of an inexpensive interferometer.
An optical imaging device in accordance with the present invention consists mainly of a light source, a light irradiating/receiving unit, a first light path member, a second light path member, a first optical branching unit, a second optical branching unit, a second light path member, a third light path member, a coupling unit, a detection unit, an optical length variation unit, and an image production unit.
The light source supplies low coherent light so that the low coherent light will be irradiated to an object and light reflected or scattered from the object will be used to construct tomographic images of the object.
The light irradiating/receiving unit irradiates the low coherent light supplied from the light source to the object, receives the light reflected or scattered from the object, and includes a first optical scanner block capable of scanning the object at least one-dimensionally in a direction of light reception or irradiation.
Over the first light path member, the low coherent light is routed to the object and the light reflected or scattered from the object is routed to the light irradiating/receiving unit.
Over the second optical path member, the low coherent light is routed.
The first optical branching unit is interposed between the light source and first optical scanner block, and branches the low coherent light supplied from the light source into the first optical scanner block and second light path member.
The second optical branching unit is incorporated in the first optical scanner block, and branches out the light reflected or scattered from the object from the first optical scanner block.
Over the third light path member, the reflected or scattered light branched out by the second optical branching unit is routed.
The coupling unit couples the low coherent light traveling over the second light path member with the reflected or scattered light traveling over the third light path member so that the low coherent light and reflected or scattered light will interfere with each other.
The detection unit detects the interference caused by the coupler to produce an interfering signal.
The optical length variation unit is coupled to one of the second and third light path members. The optical length variation unit varies at least one of a phase delay and a group delay of light by utilizing an incidence light path and an emission light path, which are mutually independent, and a light-transmissive optical element interposed between the incidence light path and emission light path. Thus, the optical length variation unit enables scanning of a point of interference in the optical-axis direction.
The image production unit processes the interfering signal detected by the detection unit to produce a tomographic image of the object.
The other features of the present invention and advantages thereof will be fully apparent from the description below.